Complementary metal-oxide semiconductor (CMOS) image sensors are ubiquitous in products that incorporate cameras, such as mobile devices, motor vehicles, medical devices, and video surveillance devices. A CMOS image sensor includes a pixel array of N×M columns and rows, respectively, wherein each pixel converts light imaged thereon by a camera lens into a digital signal that is converted into part of a displayed image and/or file containing image data.
Each pixel includes a photodiode, which is a source of photodiode reset noise that may introduce artifacts into the image data. A CMOS image sensor may include correlated double-sampling (CDS) circuits to decrease or eliminate such noise. A CDS circuit, as its name implies, samples the output from a pixel twice per sampling period: (i) a reference sample acquired after a reset transistor resets the photodiode voltage, and (ii) an image data sample acquired after a transfer gate is pulsed, which results in integrating signal charge accumulated in the photodiode. Reset noise in the image data sample is removed by subtracting the reference sample from the image data sample.
Absent significant photodiode reset noise, random telegraph signal (RTS) noise is a significant noise source. A source-follower transistor in each pixel is one source of RTS noise, which manifests in the digital signal representation as flickering display pixels in low-light conditions. A second source of RTS noise is column readout circuitry corresponding to each pixel column of the CMOS image sensor, referred to herein as column-wise RTS noise or biased RTS noise.